From mboxrd@z Thu Jan 1 00:00:00 1970 From: "H. Peter Anvin" Subject: Re: Is this enough for us to have triple-parity RAID? Date: Fri, 20 Apr 2012 12:39:17 -0700 Message-ID: <4F91BB65.8040304@zytor.com> References: <4F8D228D.8060005@westcontrol.com> <20120417171609.GA2859@lazy.lzy> <4F8DD02F.1060504@westcontrol.com> <4F905690.3060301@zytor.com> <067e21e2-6f21-48a7-93a8-bb2249534155@email.android.com> <4F91B1C4.5080205@hesbynett.no> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Return-path: In-Reply-To: <4F91B1C4.5080205@hesbynett.no> Sender: linux-raid-owner@vger.kernel.org To: David Brown Cc: Alex , linux-raid@vger.kernel.org List-Id: linux-raid.ids On 04/20/2012 11:58 AM, David Brown wrote: > Hi, >=20 > Yes, being a generator for GF(2^8) is a requirement for a parity > generator (sorry for the confusing terminology here - if anyone has a > better suggestion, please say) to be part of a 255 data disk system. > However, being a GF generator is necessary but not sufficient - using > parity generators (1, 2, 4, 16) will /not/ give quad parity for 255 d= ata > disks, even though individually each of 1, 2, 4 and 16 are generators > for GF. >=20 > 255 data disks is the theoretical limit for GF(2=E2=81=B8). But it i= s a > theoretical limit of the algorithms - I don't know whether Linux md r= aid > actually supports that many disks. I certainly doubt if it is useful= =2E >=20 > It might well be that a 21 data disk limit quad parity is useful - or= at > least, as useful as quad parity ever would be. It would fit well wit= hin > a typical large chassis with 24 disk slots. And then it doesn't matt= er > that 8 is not a generator for GF(2=E2=81=B8) - it becomes the best ch= oice > because of the easiest implementation. >=20 It is also worth noting that there is nothing magical about GF(2^8). I= t is just a reasonable tradeoff when tables are needed. There are hardware tricks one can play to do efficient operation of wider fields, too. But It sounds like {04} or {8e} are particular interesting generators o= f the existing GF(2^8) field for an efficient second field, giving triple-parity RAID again at a reasonable cost. -hpa --=20 H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- To unsubscribe from this list: send the line "unsubscribe linux-raid" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html